Additional evidence of existence of ancient rocks in the mid-Atlantic ridge and the age of the opening of the Atlantic

A metabasalt dredged at a junction of the median valley with the Atlantis fracture zone (30°01/tN, 42°04/tW) in the Mid-Atlantic Ridge shows complete recrystallization under a metasomatic condition, though the original igneous texture of a coarse-grained basalt is still recognizable. There is strong circumstantial evidence suggesting that this rock is not an ice-rafted erratic, but an authentic Mid-Atlantic Ridge rock. The ⁴⁰Ar-³⁹Ar age of this sample is 169 m.y. (Jurassic) which should represent the time of recrystallization. The initial value (^87Sr/86Sr)_O is 0.720, far above the values previously observed in oceanfloor basalts, including both tholeiitic and alkalic rocks (0.701–0.704). Sr with such a high isotopic ratio is considered to have been introduced by metasomatism during metamorphism by a solution coming from a continental mass or masses which were then located very close to the Mid-Atlantic Ridge. The ⁴⁰Ar−³⁹Ar age of sample AM50 may approximate the time of the commencement of the opening of the Atlantic. All these data support the possible existence of ancient rock masses in the Mid-Atlantic Ridge, as was formerly claimed by Bonatti, Melson and others.     

Coda Q in the Kachchh Basin, Western India Using Aftershocks of the Bhuj Earthquake of January 26, 2001

Q_C-estimates of Kachchh Basin in western India have been obtained in a high frequency range from 1.5 to 24.0 Hz using the aftershock data of Bhuj earthquake of January 26, 2001 recorded within an epicentral distance of 80 km. The decay of coda waves of 30 sec window from 186 seismograms has been analysed in four lapse time windows, adopting the single backscattering model. The study shows that Q_c is a function of frequency and increases as frequency increases. The frequency dependent Q_c relations obtained for four lapse-time windows are: Q_c=82 f^1.17 (20–50 sec), Q_c=106 f^1.11 (30–60 sec), Q_c=126f^1.03 (40–70 sec) and Q_c=122f^1.02 (50–80 sec). These empirical relations represent the average attenuation properties of a zone covering the surface area of about 11,000, 20,000, 28,000 and 38,000 square km and a depth extent of about 60, 80, 95, 110 km, respectively. With increasing window length, the degree of frequency dependence, n, decreases marginally from 1.17 to 1.02, whereas Q₀ increases significantly from 82 to 122. At lower frequencies up to 6 Hz, Q_c⁻¹ of Kachchh Basin is in agreement with other regions of the world, whereas at higher frequencies from 12 to 24 Hz it is found to be low.     

Desertification processes in the Ili River delta under anthropogenic pressure

Desertification of delta landscapes, mainly soil cover, is considered on the example of the Ili River, which basin is located in Kazakhstan and China. Long-term researches on the Ili River were begun in 1968 before creation of the large Kapchagay reservoir and proceed with the use of the space imagery NASA/MODIS/Terra. Degradation of delta landscapes is differentiated in space and in time depending on the features of the natural water regime and its transformation because of flow regulation. The main accent in research has been made on the newest processes of desertification, caused by intensification of irrigation. Characteristics of drying, desertification and salinization of soils in delta are estimated on the state of spring 2008.     

Optimization of mobile radioactivity monitoring networks

In case of a nuclear accident, decision makers rely on high-resolution and accurate information about the spatial distribution of radioactive contamination surrounding the accident site. However, the static nuclear monitoring networks of many European countries are generally too coarse to provide the desired level of spatial accuracy. In the Netherlands, authorities are considering a strategy in which measurement density is increased during an emergency using complementary mobile measuring devices. This raises the question, where should these mobile devices be placed? This article proposes a geostatistical methodology to optimize the allocation of mobile measurement devices, such that the expected weighted sum of false-positive and false-negative areas (i.e. false classification into safe and unsafe zones) is minimized. Radioactivity concentration is modelled as the sum of a deterministic trend and a zero-mean spatially correlated stochastic residual. The trend is defined as the outcome of a physical atmospheric dispersion model, NPK-PUFF. The residual is characterized by a semivariogram of differences between the outputs of various NPK-PUFF model runs, designed to reflect the effect of uncertainty in NPK-PUFF meteorological inputs (e.g. wind speed, wind direction). Spatial simulated annealing is used to obtain the optimal monitoring design, in which accessibility of sampling sites (e.g. distance to roads) is also considered. Although the methodology is computationally demanding, results are promising and the computational load may be considerably reduced to compute optimal mobile monitoring designs in nearly real time.     

Volcanic emissions and the early Earth atmosphere

Despite uncertainties in our understanding of early Earth volcanism and atmospheric composition, thermodynamic modelling is able to offer estimates of the global production of reactive trace species (NO, OH, SO₃, Cl, Br and I) from early Earth volcanism, and thereby to shed light on processes which may have been different in Earth’s early atmosphere. Model results show that thermal decomposition of magmatic H₂O, CO₂ and SO₂ in high-T mixtures of magmatic and atmospheric gases (at T > 1400 °C) generate high levels of reactive trace gas species. Production of these reactive trace species is insensitive to atmospheric CO₂ in mixtures where the atmospheric gas is the minor component and will hence continue during periods of low atmospheric CO₂. Fluxes of NO, OH, Cl, Br and I from early Earth volcanism are predicted to exceed those from modern Earth volcanism as the higher temperature of early Earth emissions compensates for lower levels of O₂ in the atmosphere, compared to the modern Earth. Under certain conditions, the volcanic NO flux from early Earth volcanism is found to be comparable to other sources of reactive N such as lightning NO and photochemical HCN. This is one possible source of fixed nitrogen which may alleviate any postulated Archean nitrogen crisis. Our thermodynamic model reveals that production of SO₃ (a potential precursor for near-source volcanic sulphate and hence ‘primary’ volcanic aerosol) is likely to be significantly lower from early Earth volcanism. Uncertainty in the pathway to near-source sulphate in modern volcanism (i.e., the reaction of SO₃ with water or direct emission) introduces a large uncertainty into the production rate of near-source volcanic sulphate on the early Earth.     

Crystal chemistry of selenates with mineral-like structures: V. Crystal structures of (H3O)2[(UO2)(SeO4)2(H2O)](H2O)2 and (H3O)2[(UO2)(SeO4)2(H2O)](H2O), new compounds with rhomboclase and goldichite topology

The crystal structures of two new compounds (H₃O)₂[(UO₂)(SeO₄)₂(H₂O)](H₂O)₂ (1, orthorhombic, Pnma, a = 14.0328(18), b = 11.6412(13), c = 8.2146(13) Å, V = 134.9(3) ų) and (H₃O)₂[(UO₂)(SeO₄)₂(H₂O)](H₂O) (2, monoclinic, P2₁/c, a = 7.8670(12), b = 7.5357(7), c = 21.386(3) Å, β = 101.484(12)°, V = 1242.5(3) ų) have been solved by direct methods and refined to R ₁ = 0.076 and 0.080, respectively. The structures of both compounds contain sheet complexes [(UO₂)(SeO₄)₂]^2? formed by cornershared [(UO₂)O₄(H₂O)] bipyramids and SeO₄ tetrahedrons. The sheets are parallel to the (100) plane in structure 1 and to (?102) in structure 2. The [(UO₂)(SeO₄)₂(H₂O)]^2? layers are linked by hydrogen bonds via interlayer groups H₂O and H₃O⁺. The sheet topologies in structures 1 and 2 are different and correspond to the topologies of octahedral and tetrahedral complexes in rhomboclase (H₂O₂)⁺[Fe(SO₄)₂(H₂O)₂] and goldichite K[Fe(SO₄)₂(H₂O)₂](H₂O)₂, respectively.     

The importance of iron speciation (Fe+2/Fe+3) in determining mineral assemblages: an example from the high‐grade aluminous metapelites of southeastern Madagascar

Metapelitic granulites from the Anosyen domain of southeastern Madagascar are exposed in three intercalated formations: the Amparihy, Bakika and Ihosy formations. Although mineralogically distinct from each other, the rocks from these formations show very similar bulk‐rock compositions when measured on a Fe_T basis. The preserved mineral assemblages thus do not reflect differences in the ratios of the main rock‐forming oxides (i.e. Al₂O₃:Fe_T:MgO), but instead reflect variations in the pre‐metamorphic oxidation state of the protolith rocks. These differences in oxidation state are manifested via differences in iron speciation – either Fe⁺² or Fe⁺³. The relatively reduced rocks of the Amparihy Formation preserve the assemblage bi–sp–sill–g–cd, which contrasts markedly with the mostly garnet and spinel‐absent bi–cd–sill–mt assemblages preserved in the strongly oxidized rocks of the Ihosy Formation. Compositionally intermediate rocks of the Bakika Formation are garnet bearing, but sillimanite‐absent, and contain the assemblage sp–g–cd–mag. Modelling of these rocks in the Na₂O–CaO–K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O–TiO₂–O system suggests that they evolved along a heating and cooling P–T path with only limited decompression accompanying cooling on the retrograde path. Peak temperatures and pressures of ～880–920 °C and 6–6.5 kbar are inferred for the majority of the Anosyen domain, with slightly lower peak temperatures (～840 °C) estimated in the extreme northwest of the area. The high‐temperature and relatively low‐pressure nature of metamorphism suggests high geothermal gradients existed during orogenesis, which in southern Madagascar is related to the amalgamation of Gondwana (580–520 Ma). Although metamorphic temperatures may have been augmented via thermal advection from the emplacement of the syn‐ to post‐tectonic Ambalavao suite, the high geothermal gradients nevertheless suggest thin and consequently hot lithosphere existed prior to orogenesis.     

Katol meteorite shower, Maharashtra: A preliminary study

A meteorite shower occurred in Katol (21° 15′ 30″ N; 78° 35′ 00″ E; at an elevation of 415 m above msl), Nagpur district of Maharashtra state, India on May 22, 2012 between 14:00 to 14:30 hrs (Indian Standard Time) with a presently observed strewn field of ～5.0 sq km area. The event was experienced by the hundreds of inhabitants with a loud noise and fire ball between Akola in the west to Nagpur in the east. The Geological Survey of India has so far collected 27 meteorite pieces with a total weight of 3500 gm. The locations, size, shape and surface features of the individual meteorite pieces have been recorded. Based on their physical properties, mineralogy, mineral chemical mapping and REE chemistry, the Katol meteorites have been classified as olivine-rich H5 type differentiated stony meteorite with reconstituted chondrules. The preliminary evidence suggests the presence of pre-solar grains in Katol meteorite.     

Volatiles from Hawaiian submarine basalts determined by dynamic high temperature mass spectrometry

Quantitative measurements of volatiles from Hawaiian submarine basalts have been made using a Knudsen cell dynamic-mass spectrometer system. The principal advantage of the technique is the ability to determine simultaneously the absolute amounts of more than one volatile released from the same sample.From mass pyrograms it was observed that the release of water was bimodal, with the major release occurring above 600°C. Water released below this temperature is believed not to have been present in the magma at the time of extrusion. Sulfur dioxide was evolved only after the bulk of the water was released and coincided with the general expansion and melting of the sample. Sulfur and carbon- containing gases which were released in surges (above 1000°C) correspond to the bursting of bubbles from the softened basalt.The molar amounts of vesicle gases were plotted as a function of extrusion depth. A change in the slope of the resulting linear curve indicates saturation of the basalt with respect to water.